Testing: Pass Or Fail?

The last two columns covered fiber optic power meters, test sources and the reference cables you need to test the loss of installed fiber optic cable plants. This month, I discuss using these instruments properly and how to determine if a tested cable plant passes or fails the test.

We know that every tech wants an instrument that simply requires the push of a button to get a pass/fail result and to record the data so they can get on to the next test. Manufacturers want that, too. Not only does it make their instruments more popular with installers, but also it reduces their training responsibilities—at least, they think so.

At The Fiber Optic Association, I field most of the technical questions. About 90 percent of the questions concern testing, and most ask us to evaluate test results. Virtually all of the questions arise because the person conductng the testing or the end-user presented with the data does not know how to evaluate it. There are two issues here. First, you have to set up the test properly, and second, you have to know what test results to expect.

Setting up the test is straightforward, as many industry standards cover it. Use your light source, power meter and reference cables (which are chosen to match the fibers and connectors in the cable plant you are testing), and set a 0-decibel (dB) loss reference. However, you have options here because standards allow three different methods of setting a 0-dB loss reference, using one, two or three reference cables. One cable is preferred because the test is more accurate, three cables can be used for special connector types, and two cable references can be used for situations where the connectors on the cables do not match the connectors on the instruments.

Adding reference cables will cause the measured loss to be lower because connections are included in setting the reference. Two cable references will measure about 0.3–0.5 dB lower than a one-cable reference, and a three-cable reference will be 0.5–1 dB lower, depending on the quality and condition of the reference cables.

So what loss do you expect? The sum of all the losses of components that make up the cable plant, what we call a “loss budget,” determines the loss of the cable plant.

Let’s start with an example of the simplest case: a single-cable run with connectors on each end. The expected loss will be the loss of the length of fiber plus the connectors on either end. We calculate the loss of the fiber by multiplying the length. Let’s use 100 meters (m) or 0.1 kilometer (km) multiplied by the estimated attenuation coefficient of that type fiber at the wavelength we are testing. For example, testing multimode fiber at 850 nanometers (nm), we can use 3 dB per km, about the average in the industry, or 3.5 dB per km, the worst-case value in TIA standards. Thus, our 100 meters of fiber should contribute a loss of 0.3 dB (0.1 km × 3.0 dB per km).

The connectors on each end are always included in a loss budget since they will have connection losses when we mate them to our reference cables. While TIA allows 0.75 dB for connectors (again, worst case), good installers should figure adhesive/polish connectors (epoxy, anaerobic or HotMelt) at about 0.3–0.5 dB and prepolished/splice connectors at 0.5–0.75 dB.

So our simple 100 m multimode cable tested at 850 nm should have a loss of:

Fiber: (0.1 km × 3.0 dB per km) = 0.3 dB

Connections: (2 each × 0.5 dB) = 1.0 dB

Total = 1.3dB

If we have more connections or even splices in a cable, we add them to the loss budget with estimated losses of splices at 0.1–0.3 dB (0.3 dB is the worst case TIA loss). On the simple cable above, if we had an intermediate connection, we would add another connection, unless the connection was done by a patchcord between the two sections of the cable when, of course, we would have to add two connections.

I use the term “connections” to describe the point of loss, not connector, because a single connector has no loss by definition unless it is mated to another connector, what it is intended to do.

Now if we test our simple cable with a meter and source, we know it has an estimated loss of 1.3 dB. Using a one-cable reference, we know a fiber in our cable that shows a tested loss of around 1.3 dB, give or take, should be good. That’s a pass. But what if we use a two- or three-cable reference method? And what’s likely to be the measurement error or uncertainty? I’ll answer those questions next month.

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